Bluetooth: Use __constant modifier for RFCOMM PSM

Since the RFCOMM_PSM is constant, __constant_cpu_to_le16() is
the right go here.

Signed-off-by: Syam Sidhardhan <s.syam@samsung.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>